At present, epitaxial silicon crystals grown on a silicon crystal substrate are widely used for devices such as ICs, LSIs, and the like. Since impurities in the epitaxial crystals greatly influence the characteristics, yields and reliability of the devices, analysis of the impurities is very important.
However, a normal epitaxial crystal is very thin, from a few .mu.m to several tens of .mu.m, and it is extremely difficult to analyze the impurities in the epitaxial crystals.
Methods for determining concentrations of impurities in crystals heretofore adopted include the resistivity method and the capacitance-voltage method.
However, both of these methods are merely applicable to the determination of concentrations of impurities most abundantly contained in the epitaxial crystals and cannot determine the kinds of the impurities. Accordingly, these methods are quite incapable of being used to assay minute amounts of residual impurities. Using the activation analysis or the secondary ion mass spectroscopy, it is possible to determine the kinds and concentrations of a number of kinds of impurities. However, the sensitivity of these methods is only about 10.sup.16 atmos/cm.sup.3 even with respect to typical impurities such as boron, phosphorus, and the like, which is far too low.
With the recent trend toward high integration and high precision in silicon devices, the importance of precise analysis of minute amounts of impurities in epitaxial crystals has increased greatly, but there has not been any known method which can directly detect various impurities in epitaxial crystals in the range of concentration less than 10.sup.16 atoms/cm.sup.3.